Method and device for spectral reconstruction of an audio signal

ABSTRACT

An audio signal encoded in the form of data is spectrally reconstructed so part of the frequency spectrum of the audio signal is decoded with a spectral band limiting encoder (i.e., a core encoder). The complementary part of the frequency spectrum of the audio signal is decoded with an extension encoder. Information representing at least one cut-off frequency of the signal decoded by the core decoder is used to select, from amongst the data to be decoded or the data decoded with the extension decoder.

The present invention concerns a method and a device for encoding anddecoding an audio signal using spectrum reconstruction techniques.

More particularly, the invention relates to improving the decoding of anaudio signal encoded by a spectral band limiting encoder, referred to asa core encoder.

In the prior art of audio signal transmission, it is well known to carryout, before transmission, an operation of encoding an original signal.As for the received signal, this undergoes a reverse decoding operation.This encoding can be a bit rate reduction encoding. Known bit ratereduction encoders are for example transform type encoders such as theMPEG1, MPEG2 or MPEG4-GA encoders, CELP type encoders and evenparametric type encoders, such as a parametric MPEG4 type encoder.

In bit rate reduction audio encoding, the audio signal must oftenundergo passband limiting when the bit rate becomes low. This passbandlimiting is necessary in order to avoid the introduction of audiblequantization noise in the encoded signal. It is then desirable tocomplete the complete spectral content of the original signal as far aspossible.

Band widening is known in the prior art, such as for example thespectral widening method known by the name HFR (High-FrequencyRegeneration) method. The decoded low-frequency signal, with limitedband, is subjected to a non-linear device in order to obtain a signalenriched with harmonics. This signal, after whitening and shaping basedon information describing the spectral envelope of the full-band signalbefore encoding, allows the generation of a high-frequency signalcorresponding to the high-frequency content of the signal beforeencoding.

Digital audio encoding systems which use high-frequency spectrumreconstruction techniques at encoder level as well as at decoder levelare also known.

These systems perform an adaptation over time of the cut-off frequencybetween the low-frequency band encoded by an encoder, referred to as thecore encoder, and the high-frequency band encoded by an HFR system,referred to as a band extension encoder.

In this case, the core encoder and the band extension encoder share thepassband according to the adapted cut-off frequency.

This type of system is particularly advantageous for encoding audiosignals.

Certain communication networks such as the Internet, wirelesscommunication networks and others do not guarantee a perfect routing ofdata between the sender and the addressee. Some data may thus neverarrive at the addressee or arrive there too late. In arriving too late,the addressee considers them as lost.

In these networks, the passband available for routing the data alsocontinuously varies considerably.

In other networks, such as radio networks, some of the data amongst thetransmitted data have a higher priority than others. Highly effectiveerror-correcting codes are associated with these, ensuring correctdecoding, and therefore no transmission losses. Others, on the otherhand, are less important and lower-performance error-correcting codes,perhaps even none, are associated with them. The latter data are subjectto the hazards of the network and decoding might well not be achievable.

In certain encoding systems such as those used in the MPEG4 standard, itmay be, following transmission errors, that the signal of a certainfrequency band of the spectrum of the encoded signal can no longer bedecoded, these frequency components then being lost.

Thus, even if the encoding of the audio signal has been performed in thebest possible manner, the decoding of signals transmitted on suchnetworks comprises a number of faults related to these networks.

The invention attempts to solve the drawbacks of the prior art byproposing a method of encoding an audio signal, in which part of thefrequency spectrum of the audio signal is encoded with a spectral bandlimiting encoder referred to as a core encoder and in which thecomplementary part of the frequency spectrum of the audio signal isencoded with an extension encoder, characterised in that at least partof the spectrum encoded with the core encoder is also encoded with theextension encoder.

Thus, at least part of the audio signal is encoded by both encoders,which guarantees correct reception of the signal, even if the latterpasses through a network in which some data may be lost or erroneous.

Correlatively, the invention proposes a device for encoding an audiosignal, in which part of the frequency spectrum of the audio signal isencoded with a spectral band limiting encoder referred to as a coreencoder and in which the complementary part of the frequency spectrum ofthe audio signal is encoded with an extension encoder, characterised inthat it comprises means for encoding at least part of the spectrumencoded with the core encoder with the extension encoder.

More precisely, determination of at least one cut-off frequency of thecore encoder is performed.

Thus, the cut-off frequency of the core encoder can be adapted to theoperating conditions of the core encoder.

More particularly, the encoded digital signal is transferred over anetwork and the or each determined frequency is transferred with theencoded digital signal.

Thus, the decoder can process this information quickly by reading itfrom the encoded digital signal.

More particularly, the core encoder is a hierarchical encoder and, foreach encoding layer, at least one cut-off frequency of each encodinglayer is determined.

Thus, for each encoding layer of the core encoder, the cut-off frequencyof the core encoder can be adapted to the operating conditions of thecore encoder.

More precisely, each encoding layer of the encoded digital signal istransferred over a network and the or each frequency determined for thelayer is transferred with said layer.

Thus, the decoder has all the information available quickly. No specialprocessing of the decoded signal is then necessary.

More precisely, the part of the spectrum encoded with the core encoderand the extension encoder is determined.

Thus, the part of the audio signal encoded by both encoders can changeover time and for example take account of the conditions of the network.

More precisely, the part of the frequency spectrum of the audio signalencoded with the core encoder is the low part of the frequency spectrumof the audio signal.

The invention also concerns a method for spectral reconstruction of anaudio signal encoded in the form of data, in which part of the frequencyspectrum of the audio signal is decoded with a spectral band limitingencoder referred to as a core encoder and in which the complementarypart of the frequency spectrum of the audio signal is decoded with anextension decoder, characterised in that the method comprises:

a step of obtaining information representing at least one cut-offfrequency of the signal decoded by the core decoder;

a step of selecting, from amongst the data to be decoded or the datadecoded with the extension decoder, data relevant for the decodingaccording to the information obtained.

Correlatively, the invention proposes a device for spectralreconstruction of an audio signal encoded in the form of data, in whichpart of the frequency spectrum of the audio signal is decoded with aspectral band limiting encoder referred to as a core encoder and inwhich the complementary part of the frequency spectrum of the audiosignal is decoded with an extension encoder, characterised in that thedevice comprises:

means for obtaining information representing at least one cut-offfrequency of the signal decoded by the core decoder;

means for selecting, from amongst the data to be decoded or the datadecoded with the extension decoder, data relevant for the decodingaccording to the information obtained.

Thus, the decoded signal will be of better quality, no spectralcomponent of the signal being absent, the frequency spectrum decodedwith the extension encoder being modified in accordance with the cut-offfrequency of the signal decoded by the core encoder.

More particularly, the part of the frequency spectrum of the audiosignal decoded with a core decoder is the low part of the frequencyspectrum of the audio signal.

Advantageously, the information representing at least one cut-offfrequency of the signal decoded by the core decoder is obtained bymaking an evaluation of the high cut-off frequency of the signal decodedby the core decoder.

Thus, it is not necessary to include additional information in theencoded and transmitted signal, and less information passes over thenetwork.

More particularly, the core decoder is a hierarchical decoder andinformation representing the passband of the signal decoded by the coredecoder is obtained for each layer of the decoded signal.

Advantageously, the information representing at least one cut-offfrequency of the signal decoded by the core decoder is obtained frominformation included in the data stream comprising the encoded digitalsignal.

Thus, the processing speed at the decoder is increased, whilstsimplifying the latter.

More particularly, the core decoder is a hierarchical decoder andinformation representing the passband of the signal decoded by the coredecoder is obtained for each layer of the decoded signal.

Thus, the decoder can adapt the processing to each encoding layer; thedecoder has this information available at each layer and can thus modifythe frequency spectrum decoded with the extension decoder according tothis information.

Correlatively, the invention proposes a signal of data representing anencoded audio signal, in which part of the frequency spectrum of theaudio signal is encoded with a spectral band limiting encoder referredto as a core encoder and in which the complementary part of thefrequency spectrum of the audio signal is encoded with an extensionencoder, characterised in that the signal comprises part of the spectrumencoded with the core encoder and with the extension encoder.

Advantageously, the signal also comprises information representing atleast one cut-off frequency of the core encoder or of the extensionencoder.

The invention also concerns the computer program stored on a datamedium, said program comprising instructions making it possible toimplement the processing method described previously, when it is loadedand executed by a computer system.

The characteristics of the invention mentioned above, as well as others,will emerge more clearly from a reading of the following description ofan example embodiment, said description being given in connection withthe accompanying drawings, amongst which:

FIGS. 1 a to 1 d depict the various frequency spectra of an audio signalencoded with a core encoder and an extension encoder;

FIGS. 1 e to 1 g depict the various frequency spectra of an audio signaltransmitted over a network and decoded with a core decoder and anextension decoder;

FIGS. 2 a to 2 e depict the various frequency spectra of an audio signalencoded with a hierarchical core encoder and an extension encoder;

FIGS. 2 f to 2 i depict the various frequency spectra of an audio signaltransmitted over a network and decoded with a hierarchical core decoderand an extension decoder;

FIGS. 3 a to 3 c depict the various frequency spectra of an audio signalencoded with a core encoder and an extension encoder according to theinvention;

FIGS. 3 d to 3 f depict the various frequency spectra of an audio signaltransmitted over a network and decoded with a core decoder and anextension decoder according to the invention;

FIG. 4 a depicts a block diagram describing the encoding deviceaccording to the invention;

FIG. 4 b depicts a block diagram describing the main elements of a corehierarchical encoder;

FIG. 5 depicts a block diagram describing the decoding device accordingto the invention;

FIG. 6 depicts, according to the invention, the algorithm performed atencoder level;

FIG. 7 depicts, according to the invention, the algorithm performed atdecoder level.

FIG. 1 a depicts a frequency spectrum of an audio signal which is to beencoded. In accordance with the encoders using combinations of encoderssuch as the core encoder/extension encoder association, the lowfrequencies of the spectrum (FIG. 1 b) are encoded by a core encoder,whilst the high frequencies are encoded by an extension encoder. Thispart of the high frequencies is depicted in FIG. 1 c.

Combining the high and low frequencies then gives a total spectrumdepicted in FIG. 1 d which is identical or else similar to the spectrumof FIG. 1 a.

When such an encoded audio signal is transmitted over a network, somedata amongst all the transmitted data are lost.

Which is for example the case of certain encoding systems such as thoseused in the MPEG4 standard. Following transmission errors, it is nolonger possible to decode the signal from a certain frequency of thespectrum of the encoded signal. The information representing thecomponents of the frequency spectrum above this frequency are thenconsidered as lost.

FIG. 1 e depicts the frequency spectrum of an audio signal decoded witha core decoder, the encoded audio signal having been transmitted over anetwork and some data 10 have been lost.

This type of loss is a particular nuisance for the information encodedby the core encoder. The absence of the data 10 constitutes a hole inthe spectrum of the decoded frequencies and this hole createssignificant noise such as hissing upon restoration of the sound signal.

The items of information encoded by the extension encoder are much morelimited as regards their number.

They are either included with the data encoded by the core encoder, ortransmitted independently.

In the example here, the frequency spectrum of an audio signaltransmitted over a network and decoded with an extension decoder isconsidered to be correct. This is depicted in FIG. 1 f.

Reconstruction of the audio signal respectively by the core decoder andthe extension decoder reveals in FIG. 1 g a frequency spectrumcomprising frequency components 10 which have disappeared.

These frequency components 10 which have disappeared considerably marthe reproduction quality of the audio signal.

FIG. 2 a depicts the frequency spectrum of the total audio signal whichis to be encoded by a hierarchical core encoder and an extensionencoder.

A hierarchical core encoder will successively encode different sub-partsof the frequency spectrum of the audio signal to be encoded.

A first part of the spectrum, for example the part containing the lowestfrequency components, such as the spectrum depicted in FIG. 2 b, will beencoded. This is referred to as the first layer. Next, another partcontaining additional frequency components will be encoded. This is thesecond layer, and is depicted in FIG. 2 c.

Thus, in such audio data transmission systems, the informationrepresenting the lowest frequencies is generally transmitted in thefirst layers. The other layers are, for example, then transmitted in anorder which is a function of the frequencies of the spectrum which theyrepresent.

In radio type data distribution networks, certain layers amongst thetransmitted layers have higher priority than others. In general, thelayers comprising the lowest frequencies are considered as havingpriority, and the layers comprising the highest frequencies areconsidered as having lowest priority.

With the layers comprising the lowest frequencies there are associatedhighly effective error-correcting codes, ensuring correct decoding, andtherefore no transmission losses.

Less effective error-correcting codes are associated with the layerscomprising the highest frequencies. The latter are subject to thehazards of the network and decoding might well not be achievable.

FIG. 2 d depicts the part of the spectrum allocated to the bandextension encoder; it is identical to that described in FIG. 1 c.

Combining the three spectra of FIGS. 2 b, 2 c and 2 d then gives a totalspectrum depicted in FIG. 2 e which is identical or else similar to thespectrum of FIG. 2 a.

FIGS. 2 f and 2 g depict the frequency spectra of an audio signaldecoded with a hierarchical core decoder comprising two layers ofhierarchy, the encoded audio signal having been transmitted over anetwork and certain layers of which have been lost.

During transmission of the first layer, the spectrum equivalent to thislayer has not been marred by transmission errors, as depicted in FIG. 2f.

Data have been lost during transmission of the second layer; thespectrum equivalent to this layer comprises frequency components, 25 inFIG. 2 g, which are absent.

The part of the spectrum allocated to the band extension encoder isidentical to that described in FIG. 1 c. It is depicted in FIG. 2 h.

Thus, reconstruction of the audio signal respectively by the corehierarchical decoder and the extension decoder reveals in FIG. 2 i afrequency spectrum comprising frequency components 25 which havedisappeared.

FIG. 3 a depicts the frequency spectrum of the total audio signal whichis to be encoded by a core encoder and an extension encoder according tothe invention.

The core encoder encodes the low-frequency components of the frequencyspectrum of the audio signal. This is depicted in FIG. 3 b.

Unlike the prior art, and according to the invention, the extensiondecoder encodes not only the high-frequency components of the frequencyspectrum of the audio signal to be encoded but also a part 30 of thelow-frequency components that the core encoder encodes. These componentsare depicted in FIG. 3 c.

FIG. 3 d depicts the frequency spectrum of an audio signal decoded witha core decoder, the encoded audio signal having been transmitted over anetwork and certain layers 31 of which have been lost.

An evaluation of the passband of the audio signal decoded by the coredecoder is made; if it is different from that expected, the core decoderinforms the extension decoder of the missing passband.

The extension decoder, with this information, adapts the decoding sothat decoding is also applied to the missing passband.

FIG. 3 e depicts the frequency spectrum equivalent to the encodedinformation received by the extension decoder. This spectrum consists ofthe components 32, 33 and 34.

If no transmission error related to variation in passband of the networkor transmission errors has occurred, the information corresponding tothe component 34 is sufficient for the decoding.

If the passband of the network has varied or transmission errors haveoccurred such that the component 31 of FIG. 3 d is lost, the informationcorresponding to the components 33 and 34 is necessary for the decoding.

Thus, reconstruction of the audio signal respectively by the corehierarchical decoder and the extension decoder reveals in FIG. 3 f afrequency spectrum no longer comprising any missing frequencycomponents. Thus, even when the network has large passband variations,the decoded audio signal remains of high quality.

FIG. 4 a depicts a block diagram describing the encoding deviceaccording to the invention.

The encoding device consists of an analogue-to-digital converter 400which converts the analogue signal to be encoded into a digital signal.Of course, if the data are already in digital form, theanalogue-to-digital converter is not necessary.

The digital signal is delivered to the core encoder which encodes thissignal. The core encoder is for example a bit rate reduction encodersuch as conforming to one of the MPEG1, MPEG2 or MPEG4-GA standards, ora CELP type encoder, a hierarchical encoder, perhaps even a parametricMPEG4 encoder.

The output of the core encoder represents the data of the signalcovering the frequency spectrum such as that depicted in FIG. 3 b.

This same digital signal is delivered to the band extension encoder 403.The band extension encoder is for example an HFR (High-FrequencyRegeneration), for example an SBR (Spectral Band Replication), typeencoder such as described in the document “Audio Engineering Society,convention paper 5553”, presented at the 112^(th) AES convention by MrMartin Dietz.

The output of the band extension encoder represents the data of theenvelope of the signal covering the frequency spectrum such as thatdepicted in FIG. 3 c.

A cut-off frequency adjustment module 402 is connected to the bandextension encoder 403 and to the core encoder 401.

This module 402 defines the frequency spectrum that the extensionencoder takes into account for the encoding.

This module 402 determines this spectrum according to the high cut-offfrequency of the core encoder 401 and a variable frequency band whichallows the decoder according to the invention to be able to overcome thepossible transmission losses.

For example, in the case of use of a hierarchical encoder andtransmission with error-correcting codes whose robustness is variableaccording to the layers transmitted, the variable frequency band isadjusted so as to guarantee correct recomposition of the signal forlayers not having a robust error-correcting code.

It should be noted that, in a variant, the frequency spectrum of thecore encoder 401 can be adjusted from the frequency spectrum of theextension encoder 403.

In this case, the module 402 defines the frequency spectrum that thecore encoder 401 takes into account for the encoding. This module 402defines this spectrum according to the low cut-off frequency of theextension encoder 403 and a variable frequency band which allows thedecoder according to the invention to be able to overcome the possibletransmission losses.

The encoding device also comprises a multiplexer 404 which multiplexesthe audio signals encoded by the core encoder 401 and by the extensionencoder 403.

According to a variant of the invention, the module 402 transfers to themultiplexer 404 the information representing the passband of the coreencoder 401 or its cut-off frequencies, perhaps even the low cut-offfrequency of the extension encoder 403, so that these are included inthe transmitted data.

The inclusion is performed in the case of a hierarchical encoder foreach encoding layer.

The multiplexed data are then transferred to a network transmissionmodule which, for example in the case of a radio transmission, applieserror-correcting codes to the multiplexed data and transmits the latterover the network 405.

FIG. 4 b depicts a block diagram describing the main elements of a corehierarchical encoder.

This hierarchical encoder can replace the encoder 401 describedpreviously with reference to FIG. 4 a.

A core hierarchical encoder usually subdivides the frequency spectrum tobe encoded into different layers. A layer represents a frequency band ofthe spectrum to be encoded. The number of layers is variable and allowsa progressive transmission of the encoded signal.

For the sake of simplicity, only two layers are depicted here. Theencoder consists of a first encoder 410 which encodes the lowest part ofthe frequency spectrum of the original signal.

The encoded information is transferred to a multiplexer 416 whichtransfers these data to the multiplexer 404.

It should be noted that the module 402 described previously transfers tothe multiplexer 404 the information representing the passband of thecore encoder 410 so that this is included in the data stream associatedwith this layer.

This then constitutes the first layer of the encoded signal.

The encoded information is also transferred to a decoder 411. Thisdecoder decodes this information in order to next transmit it to asubtraction circuit 413 which will subtract the decoded signal from theoriginal signal.

It should be noted that the original signal has previously been delayedby a time period equal to the encoding time of the encoder 410 and thedecoding time of the decoder 411.

The signal obtained at the output of the subtraction circuit is then theoriginal signal from which the previously encoded low-frequencycomponents have been removed except for the remainder of the encoding.

This signal is again encoded by an encoder 415 which may be of the sametype as the encoder 410. Here, the frequency components of the signalwhich are above those encoded by the encoder 410 are encoded.

The encoded information is transferred to a multiplexer 416 whichtransfers these data to the multiplexer 404.

It should be noted that the module 402 described previously transfers tothe multiplexer 404 the information representing the passband of thecore encoder 415 so that this is included in the data stream associatedwith this layer. It may also transfer the total number of encodinglayers, or the high or low cut-off frequency of the core encoder 415.

This then constitutes the second layer of the encoded signal.

It should be noted that, if it is wished to increase the number oflayers, the elements 410, 411, 413 and 414 must be duplicated for eachadditional layer.

It should also be noted that the frequency spectrum processed by eachencoder can be variable.

It should also be noted that the invention is applicable for audiosignals of monophonic, stereophonic or multi-channel type.

In the case of multi-channel signals, the passband informationtransmitted by the encoder can be transmitted in a combined manner or,in a preferential mode, the passband of each channel can be deduced fromthe other channels by differential encoding.

FIG. 5 depicts a block diagram describing the decoding device accordingto the invention.

The decoding device consists of a demultiplexer 510 which separates thesignals received by means of the network 405 into data intended for thecore decoder 511 and data intended for the extension decoder 512. Italso extracts, from the received signals, the information representingthe passband of the core encoder 401 of the encoding device, of theencoders 410 and 415 if the signal was encoded with a hierarchicalencoder, perhaps even the low cut-off frequency of the extension encoder403 of the encoding device, if these were included in the transmitteddata.

The core decoder 511 decodes the data in order to supply a decodedsignal such as the signal depicted in FIG. 3 d.

The core decoder 511 is for example a decoder such as conforming to oneof the MPEG1, MPEG2 or MPEG4-GA standards, or a CELP type decoder, ahierarchical decoder, perhaps even a parametric/MPEG4 decoder.

The core decoder 511 comprises a module 511 b for obtaining informationrepresenting at least one cut-off frequency which evaluates, accordingto a first embodiment, the frequency spectrum of the signal receivedthereby. The module 511 b implements this for example by performing atime-frequency transformation on the decoded signal and determining thefrequency from which the energy of the signal becomes negligible.Preferably, this can be performed with the assistance of a perceptionmodel.

The decoder 511, more precisely its module 511 b, next transfers an itemof information representing the cut-off frequency or the passband to theextension decoder 512.

The extension decoder 512 selects, using the representative item ofinformation transmitted by the decoder 511, from amongst the encodeddata it has received from the multiplexer 510, the data corresponding toa representation of the spectral envelope above the frequency determinedby the encoder 511.

In this way, the losses related to the transmission of the encodedsignal are compensated for.

The core decoder 511, more precisely the module 511 b for obtaininginformation representing at least one cut-off frequency, obtains fromthe demultiplexer 510, according to a second embodiment, the informationrepresenting the passband of the core encoder 401 or of the encoders 410and 415 of the encoding device, or perhaps the number of layers of theencoded signal, perhaps even the low cut-off frequency of the extensionencoder 403 of the encoding device, if these were included in thetransmitted data.

Using these obtained data, the module 511 b checks, in the case wherethe latter is a hierarchical decoder, whether each layer has beencorrectly received and, if not, transfers an item of informationrepresenting the passband of one or more lost layers to the extensiondecoder 512.

The extension decoder 512 selects, using the representative item ofinformation transmitted by the module 511 b, from amongst the encodeddata received from the multiplexer 510, the data corresponding to theenvelope of the signal corresponding to a representation of the spectralenvelope of the frequencies above the lowest frequency corresponding tothe lost frequency bands.

Thus, the extension decoder corrects the losses due to the networkwhether concerning losses affecting the last layers received or lossesaffecting an intermediate layer.

The band extension decoder 512 is for example an HFR (High-FrequencyRegeneration) type decoder, for example an SBR (Spectral BandReplication) type decoder such as described in the document “AudioEngineering Society, convention paper 5553”, presented at the 112^(th)AES convention by Mr Martin Dietz.

It should be noted that, in a variant, the extension decoder 512 decodesall the information received. A selection from amongst the decoded datais performed so as to keep only those corresponding to a representationof the spectral envelope above the frequency determined by the encoder511.

The envelope decoded by the extension decoder 512 or selected istransferred to a gain control module 515.

The signal decoded by the core decoder 511 is sent to a transpositionmodule 513 which generates a signal in the high frequencies of thespectrum from the low-frequency decoded signal.

This signal is introduced into the gain control module 515 in order toallow adjustment of the high-frequency signal envelope.

The adjusted envelope signal is then added to the signal decoded by thecore decoder 511 with an adder 516.

The adder 516 can in a preferred embodiment favour certain frequencycomponents by multiplying for example certain components bycoefficients.

It should be noted that the signal decoded by the core decoder 511 haspreviously been delayed by a time period equal to the difference inprocessing time between the added signals. This delay is performed bythe delay circuit 514.

The frequency spectrum of the signal obtained is thus similar to that ofFIG. 3 f.

The summation signal can next be converted into analogue form by meansof a digital-to-analogue converter 517.

FIG. 6 depicts the algorithm performed according to the invention at theencoder. The invention as described with reference to the precedingfigures can also be implemented in software form in which a processorexecutes the executable code associated with the steps E1 to E7 of thealgorithm of FIG. 6.

Upon power-up of the encoding device, and more particularly in the caseof use of a computer as the encoding device, the processor reads, fromthe read-only memory of the computer or from a data medium such as acompact disk (CD-ROM), the instructions of the program corresponding tothe steps E1 to E7 of FIG. 6 and loads them into random access memory(RAM) in order to execute them.

At the step E1, upon receipt of audio data to be encoded, the processordetermines the passband of the core encoder or at least one cut-offfrequency.

It should be noted that the passband of the core encoder may or may notbe variable over time depending for example on the load of the coreencoder.

At this same step, the processor encodes the data according to aso-called core encoding algorithm conforming to one of the MPEG1, MPEG2or MPEG4-GA standards, or of CELP type, of hierarchical type, perhapseven of parametric MPEG4 type.

The step E2 consists of checking whether, and in the case ofhierarchical encoding, all the layers have been encoded or not.

If not, and if the core encoding is a hierarchical encoding, theprocessor reiterates the step E1 for each layer of the encoded audiosignal.

If all the layers have been encoded, or if the encoding is not ahierarchical encoding, the algorithm goes to the next step E3.

At the step E3, the processor determines a frequency margin. This marginmay be predetermined and stored in a register or be in the form of avariable.

This variable depends for example on the type of error correction whichwill be applied to the encoded data during their transmission over thenetwork.

This margin having been determined, the processor determines at the stepE4, from the margin and the high cut-off frequency of the core encoder,the low cut-off frequency of the extension encoder.

This operation having been carried out, the processor transfers thisinformation to the extension encoding subroutine at the step E5.

Finally, according to a particular embodiment of the invention, at thestep E6, the processor stores this information.

The processor, at the step E7, executes the extension encoding byencoding the data whose spectrum is above the information transferred atthe step E5. The band extension encoding is for example an encoding ofthe HFR (High-Frequency Regeneration), for example SBR (Spectral BandReplication), type such as described in the document “Audio EngineeringSociety, convention paper 5553”, presented at the 112^(th) AESconvention by Mr Martin Dietz.

This operation having been performed, the processor goes to the step E7which consists of multiplexing the audio signals encoded at the step E1and the audio signals encoded at the step E7 in order to form a streamof data encoded and transmitted over a network.

According to a variant of the invention, the processor inserts, into theencoded and transmitted data stream, the information stored at the stepE6 or inserts one or more of the following items of information:passband of the core encoder, passband of the extension encoder, low andhigh frequency of each encoding layer, number of encoding layers if ahierarchical encoder is used.

The insertion is performed in the case of a hierarchical encoder foreach encoding layer.

These operations having been performed, the processor returns to thestep E1 awaiting new audio data to be encoded.

FIG. 7 depicts the algorithm performed according to the invention at thedecoder.

The invention as described with reference to the preceding figures canalso be implemented in software form in which a processor executes thecode associated with the steps E10 to E15 of the algorithm of FIG. 7.

Upon power-up of the receiving device, and more particularly in the caseof use of a computer as the receiving device, the processor reads, fromthe read-only memory of the computer or from a data medium such as acompact disk (CD-ROM), the instructions of the program corresponding tothe steps E10 to E15 of FIG. 7 and loads them into random access memory(RAM) in order to execute them.

At the step E10, the processor, upon receiving audio data to be decoded,separates the signals received by means of the network 405 into dataintended for the core decoder and data intended for the extensiondecoder. It also extracts, from the received signals, the informationrepresenting the passband or at least one cut-off frequency of the coreencoder which encoded the audio signal, or of the encoders which encodedthe audio signal if the signal was encoded with a hierarchical encoder,perhaps even the low cut-off frequency of the extension encoder whichencoded the audio signal, if these were included in the transmitteddata.

This operation having been performed, the processor goes to the stepE11. The processor then carries out the decoding of these data.

The processor carries out the decoding of the data according to aso-called core decoding algorithm such as conforming to one of theMPEG1, MPEG2 or MPEG4-GA standards, or of CELP type, a hierarchicaldecoding, perhaps even a parametric MPEG4 type decoding.

This core decoding step having been performed, the processor goes to thestep E12 which is a step of obtaining information representing at leastone cut-off frequency which evaluates, according to a first embodiment,the frequency spectrum of the signal received thereby. This is carriedout for example by performing a time-frequency transformation on thesignal decoded at the step E11 and determining the frequency from whichthe energy of the signal becomes negligible. Preferably, this can beperformed with the assistance of a perception model.

According to another embodiment, the processor obtains the informationextracted at the step E1 and, in the case where the latter is ahierarchical decoder, checks whether each layer has been correctlyreceived and if not transfers an item of information representing thepassband of one or more lost layers to the extension decoder.

This operation having been performed, the step E13 consists of anadaptation of the low cut-off frequency of the extension decoder so thatthe latter compensates for the losses due to the network. The adaptationis performed using the information representing the cut-off frequency orthe passband obtained at the step E12 or, if the decoding of the stepE11 is a hierarchical decoding, the information representing thepassband or a cut-off frequency of one or more lost layers.

This operation having been performed, the processor goes to the step E14and, according to a so-called extension decoding algorithm, decodes thedata corresponding to the frequencies above this previously determinedlow cut-off frequency.

The processor selects, using the adapted frequency, from amongst thedata separated at the step E1 and intended for the extension decoding,the data corresponding to the envelope of the signal corresponding to arepresentation of the spectral envelope of the frequencies above thelowest frequency corresponding to the lost frequency bands.

Thus, the extension decoding corrects the losses due to the network,whether concerning losses affecting the last layers received or lossesaffecting an intermediate layer.

The extension decoding is a band extension decoding algorithm forexample an HFR (High-Frequency Regeneration) type decoding, for examplean SBR (Spectral Band Replication) type decoding such as described inthe document “Audio Engineering Society, convention paper 5553”,presented at the 112^(th) AES convention by Mr Martin Dietz.

Finally, the data decoded by the core decoder and the extension decoderare added to form the decoded audio signal at the step E15.

These operations having been performed, the processor returns to thestep E10 awaiting new audio data to be decoded.

1. Method of encoding an audio signal, in which part of the frequencyspectrum of the audio signal is encoded with a spectral band limitingencoder referred to as a core encoder and in which the complementarypart of the frequency spectrum of the audio signal is encoded with anextension encoder, wherein at least part of the spectrum encoded withthe core encoder is also encoded with the extension encoder, the methodcomprising: determining at least one cut-off frequency of the coreencoder; determining the part of the spectrum encoded with the coreencoder and the extension encoder using the determined cut-offfrequency.
 2. Method according to claim 1, wherein the method comprisestransferring the encoded digital signal over a network and transferringthe or each determined frequency with the encoded digital signal. 3.Method according to claim 1, wherein the core encoder is a hierarchicalencoder and, for each encoding layer, at least one cut-off frequency ofeach encoding layer is determined.
 4. Method according to claim 3,wherein the method comprises transferring each encoding layer of theencoded digital signal over a network, transferring the or eachdetermined frequency for the layer with said layer.
 5. Method accordingto claim 1, wherein the part of the frequency spectrum of the audiosignal encoded with the core encoder is the low part of the frequencyspectrum of the audio signal.
 6. Method of spectral reconstruction of anaudio signal encoded in the form of data, in which part of the frequencyspectrum of the audio signal is decoded with a spectral band limitingdecoder, referred to as a core decoder, and in which the complementarypart of the frequency spectrum of the audio signal is decoded with anextension decoder, the method comprising: obtaining informationrepresenting at least one cut-off frequency of the signal decoded by thecore decoder; selecting, from amongst the data to be decoded or the datadecoded with the extension decoder, data relevant for the decodingaccording to the obtained information.
 7. Method according to claim 6,wherein the part of the frequency spectrum of the audio signal decodedwith a core decoder is the low part of the frequency spectrum of theaudio signal.
 8. Method according to claim 6, wherein the informationrepresenting at least one cut-off frequency of the signal decoded by thecore decoder is obtained by making an evaluation of the high cut-offfrequency of the signal decoded by the core decoder.
 9. Method accordingto claim 6, wherein the information representing at least one cut-offfrequency of the signal decoded by the core decoder is obtained frominformation included in the data stream comprising the encoded digitalsignal.
 10. Method according to claim 8, wherein the core decoder is ahierarchical decoder and the method obtains information representing thepassband of the signal decoded by the core decoder for each layer of thedecoded signal.
 11. Device for encoding an audio signal, in which partof the frequency spectrum of the audio signal is encoded with a spectralband limiting encoder, referred to as a core encoder, and in which acomplementary part of the frequency spectrum of the audio signal isencoded with an extension encoder, the device comprising: means fordetermining at least one cut-off frequency of the core encoder; meansfor determining the part of the spectrum encoded with the core encoderand the extension encoder using the determined cut-off frequency, meansfor encoding at least part of the spectrum encoded with the core encoderwith the extension encoder.
 12. Device according to claim 11, whereinthe device comprises means for transferring the coded digital signalover a network and for transferring the or each determined frequencywith the encoded digital signal.
 13. Device according to claim 11,wherein the core encoder is a hierarchical encoder arranged fordetermining, for each encoding layer, at least one cut-off frequency.14. Device according to claim 14, wherein the device comprises means fortransferring each layer of the encoded digital signal over a network andfor transferring the or each frequency determined for the encoding layerwith said encoding layer.
 15. Device according to claim 11, wherein thepart of the frequency spectrum of the audio signal encoded with the coreencoder is the low part of the frequency spectrum of the audio signal.16. Device for spectral reconstruction of an audio signal encoded in theform of data, in which part of the frequency spectrum of the audiosignal is decoded with a spectral band limiting decoder referred to as acore decoder and in which the complementary part of the frequencyspectrum of the audio signal is decoded with an extension encoder, thedevice comprising: means for obtaining information representing at leastone cut-off frequency of the signal decoded by the core decoder; meansfor selecting, from amongst the data to be decoded or the data decodedwith the extension decoder, data relevant for the decoding according tothe information obtained.
 17. Device according to claim 16, wherein thepart of the frequency spectrum of the audio signal decoded with a coredecoder is the low part of the frequency spectrum of the audio signal.18. Device according to claim 16, wherein the information representingthe passband of the signal decoded by the core decoder is arranged to beobtained by making an evaluation of at least one cut-off frequency ofthe signal decoded by the core decoder.
 19. Device according to claim16, wherein the information representing at least one cut-off frequencyof the signal decoded by the core decoder is arranged to be obtainedfrom information included in the data stream comprising the encodeddigital signal.
 20. Device according to claim 19, wherein the coredecoder is a hierarchical decoder and the device is arranged forobtaining information representing at least one cut-off frequency of thesignal decoded by the core decoder for each layer of the decoded signal.21. A data medium storing a computer program, said program comprisinginstructions making it possible to implement the encoding methodaccording to claim 1, when the program is loaded and executed by acomputer system.
 22. A data medium storing a computer program, saidprogram comprising instructions making it possible to implement theaudio signal reconstruction method according to claim 6, when theprogram is loaded and executed by a computer system.
 23. A processorarrangement arranged to perform the steps of claim
 1. 24. A processorarrangement arranged to perform the steps of claim 6.